This invention relates to the recovery of nickel values from an aqueous solution by means of a liquid/liquid extraction process employing certain alpha,beta-dioxime compounds as selective extraction agents. This invention also relates to novel alpha, beta-dioxime compounds.
One of the methods which can be used for accomplishing the recovery of nickel from ores comprises grinding the ore and then leaching the ground ore with an acidic aqueous solution. An acidic aqueous leachate is thus obtained which contains various metal ions. Selective recovery of nickel values from the leachate is subsequently achieved by contacting the aqueous metal value-containing solution with an organic extractant comprising an organic solvent and an extraction agent. The extraction agent is a ligand which forms chelate complexes with the nickel values, but preferably does not complex other metal values, such as cobalt and iron, which may also be present in the aqueous leachate. In such a process, the ligand and the ligand/nickel complexes must both be appreciably soluble in the organic solvent of the extractant, but not in water. Contact of the organic extractant with the aqueous leachate thus results in the selective transfer of nickel values into the extractant solution.
To be successful in application to commercial nickel extraction processes, the ligands must be able to complex with the nickel values both rapidly and completely. The rate of transfer of nickel values from an aqueous to organic phase in such an extraction process is conventionally expressed in terms of "extraction yield," which is defined by means of the following illustration. When an aqueous solution containing nickel values has been contacted, with agitation, for a period of, say, "t" minutes with the organic extractant, a sample is drawn from the mixture while agitation is continued. This sample is allowed to separate into an aqueous phase and an organic phase and the nickel content of the organic phase--indicated as "a" mg Ni/l--is determined. When, after prolonged agitation of the aqueous layer with the organic solution, equilibrium has been reached, the mixture is separated into an aqueous phase and an organic phase and the nickel content of the organic phase and an organic phase and the nickel content of the organic phase--indicated as "b" mg Ni/l--is determined. The extraction yield after t minutes' agitation, expressed as a percentage, is defined as EQU a/b.times.100.
The extent to which nickel values in the aqueous solution are transferred into the extractant, expressed in terms of a "distribution coefficient", is also of importance. The distribution coefficient after t minutes' agitation is defined as EQU a/c,
wherein "a" is as defined hereinbefore and "c" is the nickel content of the aqueous phase, expressed in mg Ni/l. To be attractive for commercial nickel value extraction processes, a ligand must be able to produce both high extraction yields after a short extraction time and high distribution coefficients.
It is generally known to employ alpha,beta-dioxime compounds as ligands in liquid/liquid extraction processes for metal value recovery. However, known alpha,beta-dioximes have proven deficient for one or more reasons when applied for purposes of selective nickel value recovery. For instance, the process for nickel value extraction utilizing 4-alkyl-1,2-cyclohexanedione dioximes, as disclosed in U.S. Pat. No. 3,703,573 to G. C. Blytas, is concerned with simultaneous extraction of copper, cobalt, and nickel from an ore leachate. Selectivity with regard to nickel value recovery in such a process is provided by the formation of dioxime/nickel value complexes which are insoluble in the aqueous and the organic extraction phases. For selective recovery of only nickel values, it is most desirable that the extraction process yield a solution of the dioxime/nickel value complexes in the organic extraction phase, and not a precipitate. Otherwise the formation of a third, solid phase during liquid/liquid extraction complicates downstream processing operations for the eventual release of the nickel from the dioxime complexes. All but one of the dioxime ligands disclosed in the Blytas patent form insoluble nickel complexes. Furthermore, the distribution coefficients obtained using the extraction agents of U.S. Pat. No. 3,703,573 are unacceptable in a commercial sense, for it is shown that the percent of nickel values extracted is quite low in acidic extraction systems, i.e. 15% at pH=2, 58% at pH=4, and 80% at pH=6. In view of the fact that in commercial operations an acidic aqueous solution is generally used to initially leach nickel values from ores, there are definite advantages in being able to extract the nickel values from this leachate at low pH. If large amounts of alkaline matter must be added to the leachate for adjustment and control of the pH prior to extraction, cost and complexity of the process are thereby increased. The 4-alkyl-1,2-cyclohexanedione dioximes of U.S. Pat. No. 3,703,573 were also found to form complexes with both cobalt and iron at low pH; selective recovery of nickel values in the presence of cobalt and iron values is thus not possible when such ligands are employed in the extraction process.
A more recent investigation of the use of alpha,beta-dioximes for nickel value extraction is that of A. R. Burkin and J. S. Preston (J. Inorg. Nucl. Chem., 1975, Vol. 37, pp. 2187-2195), concerning symmetrical alpha,beta-alkanedione dioximes. These dioximes were found to provide for more selective extraction of nickel over cobalt and iron, and the dioxime/nickel complexes were soluble in the organic extractant. However, formation of cobalt (II) complexes was found to be irreversible. Furthermore, extraction of nickel values proceeded very slowly at low pH.
In view of the deficiencies of known alpha,beta-dioximes when applied to such liquid/liquid extraction processes, a dioxime compound able to provide for selective recovery of nickel in an extraction process characterized by high extraction yield after a short extraction time and high distribution coefficients at low pH would be highly desirable.